The present invention pertains to a polystyrene polymer exhibiting a new set of characteristics. These characteristics permit a high degree of expandability if the polymer is used in admixture with a blowing agent. The present invention also pertains to a formulation of the highly-expandable polymer in combination with a low level of blowing agent. The high degree of expandibility of the polymer of the present invention in turn permits the use of a low level of blowing agent therewith, resulting in the ability to utilize the formulation of the present invention in the production of an expanded polystyrene product of low density (e.g. 0.8 to 1.1 lb./cu.ft.) with blowing agent present in an amount of from just 2 to 5.5 weight percent, based upon the total weight of the formulation.
Typical commercial production of expanded polystyrene products has utilized only "one pass" expansion processes. ["One pass" prosesses are those which pre-expand styrene particluates only once before the molding operation.] These one-pass expansion processes required the use of a blowing agent in an amount of from about 5 weight percent to about 8 weight percent for the production of expanded polystyrene products having a density of from about 0.9 to about 1.1 lb./cu.ft.
In recent years volatile organic compound (VOC) air emissions have come under increasing scrutiny by the EPA, state, and local air quality boards as mandated by the Clean Air Act of 1977. Because hydrocarbon emissions have been shown to contribute to photochemical smog, the expanded polystyrene industry which uses pentane as a blowing agent has come under pressure to limit its use and/or emissions of pentane.
The inventors of the present invention unexpectedly discovered a highly-expandable polymer which they have characterized. The inventors also discovered a formulation (comprising the highly-expandable polymer in conjunction with a low level of blowing agent) which can be used to produce an expanded polystyrene product having a density of from about 0.8 lb./cu.ft. to about 1.1 lb./cu.ft. when the formulation (existing in the form of solid particles) is expanded in from 2 to 5 expansion steps (i.e. a "multipass" process). As a result, the polymer (and formulation utilizing same) provides the heretofore unavailable advantage of utilizing a very low level of blowing agent (most preferably about 3.5 weight percent, based on the total formulation weight) which thereby results in the release of a heretofore unachievable low level of blowing agent into the environment during the expansion process.
Since the early months of 1990, the inventors' polymer and formulation has enjoyed a high level of commercial success, with sales of at least 3 million pounds thereof, which formulation has the highly-expandable polymer present in an amount of about 96 weight percent, based on the total weight of product. Thus there has been a high level of commercial success of both the polymer as well as the formulation utilizing the polymer.
The inventors of the present invention are aware of several related art documents and products which are discussed below. First, U.S. Pat. No. 4,520,135 and a divisional application thereon (which issued as U.S. Pat. No. 4,525,484) describes a polystyrene particles containing blowing agent and having improved expandability based on polystyrene having a weight-average mean molecular weight of not more than from 130,000 to 180,000 and a molecular weight distribution curve in which the high molecular weight side of the curve drops more steeply. These expandable polystyrene particles were produced by polymerizing styrene in aqueous suspension in the presence of blowing agents and of chain transfer agents or styrene oligomers.
In contrast to the '135 and '484 patents, the polymer of the present invention as well as the polymer within the formulation of the present invention have both: (1) a substantially lower polydispersity; as well as (2) a substantially higher weight average molecular weight, in comparison with the polymers described in the '135 and '484 patents. It has surprisingly been found that even though the weight average molecular weight of the polymer of the present invention is higher than the weight average molecular weight of the '135 and '484 patents, the polymer of the present invention has a very desirable high degree of expandability and moldability.
Other related documents include: U.S. Pat. No. 2,884,386; U.S. Pat. No. 4,839,396; U.S. Pat. No. 4,485,193; U.S. Pat. No. 3,639,551; U.S. 3,631,133; U.S. Pat. No. 3,589,769, U.S. Pat. No. 3,126,432, U.S. Pat. No. 3,056,753; and U.S. Pat. No. 4,721,588. None of these patents warrants any detailed discussion since none discloses subject matter which is very close to the polymer and formulation of the present invention.
For several years BASF Corporation has been involved in the manufacture and sale of a number of expandable polystyrene formulations having approximately 6 weight percent pentane therein. Typically these formulations contained a polymer having a polydispersity of 2.2, a weight average molecular weight of about 190,000, and an M.sub.z :M.sub.n of about 3.5. In stark contrast, the product of the present invention has a polydispersity of from 1 to less than 2, a weight average molecular weight of from about 200,000 to about 300,000, and an M.sub.z :M.sub.n of from about 2 to less than 3.
One polymer which has been commercialized for several years has a polydispersity of about 1.9, a weight average molecular weight of about 190,000, and furthermore, upon analysis, yielded an M.sub.z :M.sub.n of 3.04. Furthermore, this polymer was produced only in formulations bearing blowing agent in an amount of about 6 weight percent. In contrast, the polymer of the present invention has a combination of characteristics (polydispersity, weight average molecular weight, and M.sub.z :M.sub.n), which differs from the aforementioned commercially available polymer. Furthermore, the formulation of the present invention utilizes blowing agent in an amount of only from about 2 weight percent to about 5.5 weight percent.